1. Field of the Invention
The present invention relates to a casing, in which a gas container is to be accommodated, i.e. a cylinder cabinet. More specifically, the present invention relates to a gas leakage detecting apparatus provided with such a cylinder cabinet.
2. Description of the Prior Art
A special material gas for use in the manufacture of semiconductors is usually used as stored in a portable gas container which is called a gas bomb or gas cylinder. As to such special material gases for use in the manufacture of semiconductors, there are many dangerous gases having combustibility (explosiveness), toxicity, corrosiveness or combustion-supporting properties, and hence they must be given with thoughtful consideration in safety when used. Accordingly, a gas container filled with a dangerous special material gas has been hitherto accommodated in a casing, which is referred to as a cylinder cabinet, thereby coping with an unforeseen gas leakage.
The cylinder cabinet itself functions as a closed box for preventing a gas from spreading to the atmosphere, the inside of which is normally ventilated so that a leaked gas can be diluted and exhausted. Its design is such that air is taken in by an air take-in port in the lower portion of the cylinder cabinet and the same air is discharged to the atmosphere through an exhaust duct in the upper portion thereof. Furthermore, said exhaust duct has a gas leakage detecting apparatus connected thereto, whereby it is possible to monitor for leaked gas.
Although one is obligated to carry out normal exhaust in a cylinder cabinet in accordance with the related laws, it is very important for safety reasons that such normal exhaust is carried out at a proper ventilation ratio or ventilation quantity depending on the kind of a gas. In the case of a combustible gas such as monosilane, for instance, it is necessary to carry out ventilation by introducing air in an amount equal to 10 times the volume of the cylinder cabinet every minute. When such a large volume of air is used, it becomes difficult to detect a gas leakage even if it takes place.
On the other hand, it is known from experience that a gas leakage seldom takes place during the usual supply of a gas, rather it takes place, in many cases, upon the exchange of a gas container or just after the exchange. Namely, the gas leakage will take place where the closing of the valve of a gas container (which will be hereafter called "a container valve") is unsatisfactory during the separation of a container valve and a pipe is carried out in order to exchange a gas container, or where the actual gas purge is started up in spite of the fact that the connection of the container valve and the pipe is unsatisfactory after the gas container exchange is completed. Also as to the place where a gas leakage occurs, it is known that such place is generally a specific given place, for example, a removable pipe joint between a gas container and a pipe, or a valve or a pressure reduction valve in a pipe.
Although the time when a gas leakage can occur and the location where it takes place are roughly understood, according to the gas leakage detecting apparatus of the prior art, a good detection result may not be obtained because the apparatus is constructed such that a gas leakage is detected by sampling air in an exhaust duct.
In consideration of this problem, it is an object of the present invention to provide a gas leakage detecting apparatus, by which the accuracy of detecting gas leaks can be improved.